1. Field of the Invention
The present invention relates to a process for producing a cold-rolled high strength steel sheet having an excellent formability and conversion-treatability (phosphatability and paintability), more specifically to a cold-rolled steel sheet having a high tensile strength of 40 kgf/mm.sup.2 or more, a high ductility, and an excellent deep-drawability and conversion-treatability including a post-lacquering corrosion resistance (corrosion resistance of a lacquered product), and therefore suitable for use as panel members such as the outer or inner panels of automobiles, which require a good press-formability, particularly a deep-drawability, and a high strength for constructing lightweight automobiles.
2. Description of the Related Art
Steel sheet used as the panels of automobiles must have a high strength, from the viewpoint of energy saving; an excellent formability, to cope with increasing variations of automobile design; and an excellent conversion-treatability due to the necessity for a lacquering thereof.
Conventional processes for producing a cold-rolled steel sheet having an excellent deep-drawability include those disclosed, for example, in Japanese Unexamined Pat. Publication (Kokai) Nos. 61-276927.sup.(1) and 61-276930.sup.(2), which ensure a high ductility and deep-drawability by a coaddition of titanium and niobium in steel for fixing carbon, nitrogen, and sulfur to purify the inside of crystal grains.
An improvement of the conversion-treatability is described in Japanese Unexamined Pat. Publication (Kokai) Nos. 61-276951.sup.(3), 57-161035.sup.(4), 61-276925.sup.(5), 61-276926.sup.(6), and 58-100622.sup.(7).
The above publications (1) and (2) specify the heating and cooling conditions during continuous-annealing to improve the post-forming toughness (toughness of a formed product) but evaluate the property at a relatively high test temperature of -20.degree. C., and therefore, the evaluation is not sufficiently strict. Namely, the improvement of post-forming toughness brought about by control of the heating and cooling conditions for the continuous-annealing is limited, and a further improvement cannot be derived therefrom. These publications disclose a steel sheet having a tensile strength of substantially 40 kgf/mm.sup.2 or less and, therefore, do not provide a process suitable for producing a steel sheet having a tensile strength of 40 kgf/mm.sup.2 or more. Moreover, when a steel sheet is to be strengthened by phosphorus and silicon additions, these publications do not disclose a way of preventing an impairment of the post-forming toughness due to the phosphorus addition or an impairment of the conversion-treatability, including the post-lacquering corrosion resistance, due to the silicon addition, although they ensure an improved strength and formability. A conversion-treatability including a post-lacquering corrosion resistance is indispensable for a steel sheet to be used in particular, as the outer panels of automobiles.
Although the publication (3) discloses a good formability and a good conversion-treatability of a silicon-added steel sheet obtained by a calcium addition, in practice it describes only a steel sheet having a silicon content of 0.02 wt % or less and a tensile strength of 32 kgf/mm.sup.2 or less; i.e., it does not disclose an improvement of the conversion-treatability of a high strength steel sheet having a good formability, and further, does not mention the problem of post-lacquering corrosion resistance.
The publication (4) discloses the coating of a sulfur compound on a silicon-containing steel sheet, and mentions the problem of the post-lacquering corrosion resistance. But the treatment thereof is for a low-carbon steel containing 0.04 wt % or more carbon, and the sulfur compound coating is intended to eliminate the adverse effect of the surface carbon on the conversion-treatability. Namely, it does not discuss an ultra-low carbon steel sheet having a formability required for steel sheets for automobiles.
The publication (5) relates to an ultra-low carbon steel sheet having a specified manganese/sulfur ratio but does not relate to a high strength steel sheet, since it discusses only a silicon content of 0.1 wt % or less and a phosphorus content of 0.01 wt % or less.
The publications (6) and (7), which relate to an ultra-low carbon steel sheet and a high strength steel sheet, respectively, are disadvantageous in that a control of the dew point of the atmosphere during annealing is required.
As described above, although improvements of the formability of a high strength steel sheet and the conversion-treatability, including post-lacquering corrosion resistance, of a relatively low strength steel sheet have been established separately, the co-existence of these properties in a high strength steel sheet has not been realized due to the technological difficulties involved.